This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2001-249048, filed Aug. 20, 2001, the entire contents of this application are incorporated herein by reference.
This invention relates to a method for laser isotope separation and enrichment of silicon on the basis of infrared multiple-photon dissociation of silicon halides, particularly to a method in which the selectivity for and yield of isotope to be separated and enriched are remarkably improved by irradiating silicon halides synchronously with two or more infrared pulsed laser beams at different wavelengths.
Non-radioactive stable isotopes exist for many elements and their use is being expanded since there is no fear of potential risk for radiation exposure.
Natural silicon consists of isotopes of mass numbers 28, 29 and 30 in the abundance ratios of 92.2% (28Si), 4.7% (29Si) and 3.1% (30Si).
Separated and enriched stable silicon isotopes have already been used in various fields and their applications in future have also been proposed. Namely, 28Si and 30Si have been used as tracers in studies on the effect of silicon fertilizers on rice and 30Si has also been used in the production of novel nuclides. The other isotope 29Si has been used in the ion implantation processes for semiconductor fabrication. In the past few years, the single crystal of high-purity 28Si has been demonstrated to show higher thermal conductivity than that of natural silicon and semiconductor chips using it are considered to permit faster heat dissipation; therefore, the single crystal of high-purity 28Si is drawing much attention as a promising candidate that contributes to further increase in the packaging density and operating speed of integrated circuits. As a potential application of single crystal silicon that is totally free of 29Si having a nuclear spin, namely, the single crystal of high-purity 28Si, the idea of the quantum computer using the magnetic properties of dopant phosphorus has been proposed and met with growing interest.
Thus, silicon isotopes have a greater possibility for large-scale use in various fields in the near future; however, in order to meet the demand for expanded use of silicon isotopes, it is essential to develop a technique capable of high-efficient isotope separation and enrichment.
Separation and Enrichment of Silicon Isotopes by Methods other than the Laser-Assisted Technology
An electromagnetic mass separator has long been used to separate silicon isotopes. In this method, however, the yield cannot be increased to a practical level by using even a much larger separator and the desired isotopes cannot be supplied in large enough quantities at low cost.
In Russia, gigantic centrifugal separators originally developed as a component of military nuclear facilities have been diverted to the purpose of silicon isotope separation; however, such gigantic centrifugal separators require huge initial investment and cannot be operated by the private sector without touching the sensitive issue of military secret.
Separation and Enrichment of Silicon Isotopes by the Laser-Assisted Technology
In order to separate and enrich silicon isotopes, the laser-assisted method has been proposed that is based on a carbon dioxide laser induced isotopically selective infrared multiple-photon dissociation of Si2F6 (Japanese Patent H2-56133, U.S. Pat. No. 4,824,537, EU Patent 0190758, Japanese Patent H5-80245, and S. Arai, H. Kaetsu and S. Isomura, Appl. Phys., B32, 199 (1991)). On the pages that follow, the laser-assisted method has been described in details and then their several problems have been pointed out.
[Infrared Multiple-Photon Dissociation of Si2F6]
When molecules are irradiated with intense laser beam in a region within their infrared absorption bands, the individual molecules may sometimes be dissociated by absorbing as many as several tens of photons. This phenomenon is called infrared multiple-photon dissociation. FIG. 1 shows an infrared absorption spectrum for a silicon fluoride Si2F6 and emission lines from a carbon dioxide laser. As shown, Si2F6 has an infrared absorption band in the emitting region of the carbon dioxide laser due to the stretching of the Sixe2x80x94F bond. If one of the emission lines in the 10R- or 10P-branch of the carbon dioxide laser is selected and Si2F6 is irradiated with its intense pulsed laser light (hv) the individual molecules of Si2F6 absorb a large number of photons to become excited in a highly vibrational state and are eventually decomposed into SiF2 and SiF4. This is what is commonly called the infrared multiple-photon dissociation of Si2F6 and described by the following scheme, where n refers to the number of laser photons absorbed by the Si2F6 molecule:
Si2F6+nhvxe2x86x92SiF2+SiF4
One of the two decomposition products, SiF4 is a stable gas molecule whereas SiF2 is an unstable molecule which undergoes cascade polymerization to produce a white solid substance having the composition (SiF2)m in accordance with the following scheme, where m refers to the number of the SiF2 molecules being polymerized:
mSiF2xe2x86x92(SiF2)m
Studies on pyrolysis have shown that the process of Si2F6 decomposition into SiF2 and SiF4 requires energy of about 190 kJ/mol. This value means that the molecule of Si2F6 that has undergone infrared multiple-photon dissociation must have absorbed at least 17 or more photons from the carbon dioxide laser.
[Absorption Spectrum and Isotope Shift]
The infrared absorption spectrum of Si2F6 shown in FIG. 1 was measured with an infrared spectrophotometer. In the ordinary measurement using the spectrophotometer, the intensity of incident light is low and an absorption spectrum corresponds to the process in which a molecule of interest absorbs one photon. The spectrum corresponding to this single-photon absorption is hereunder referred to as the ordinary infrared absorption spectrum.
In the ordinary infrared absorption spectrum of Si2F6, the peak of the absorption band due to the stretching of the Sixe2x80x94F bond is located at 990 cmxe2x88x921 for 28Sixe2x80x94F, at 982 cmxe2x88x921 for 29Sixe2x80x94F and at 974 cmxe2x88x921 for 30Sixe2x80x94F. An effect generally called the isotope shift is often observed between absorption bands for different isotopes. Since the peaks of the absorption bands for 28Sixe2x80x94F, 29Sixe2x80x94F and 30Sixe2x80x94F in FIG. 1 are located so close to each other that adjacent bands overlaps, and, in addition, the amounts of 29Sixe2x80x94F and 30Sixe2x80x94F are much smaller than that of 28Sixe2x80x94F, it is difficult to distinguish between the peak positions except for 28Sixe2x80x94F.
Each Si2F6 molecule absorbs at least 17 photons and undergoes infrared multiple-photon dissociation. As is well known, the multiple-photon absorption spectrum which corresponds to the process of molecular absorption of many photons differs from the ordinary single-photon absorption spectrum shown in FIG. 1 and the peak positions of the absorption bands shift toward a longer wavelength side or toward a smaller wave number side, and the widths of the absorption bands become broader. The occurrence of isotope shifts comparable to those in the single-photon absorption spectrum is also predicted for the multiple-photon absorption spectrum and, in fact, has been demonstrated in experiments.
[Principle of Laser Isotope Separation and Enrichment]
Noting the difference in infrared multiple-photon absorption that is observed between molecules containing different isotopes, one can irradiate such molecules with laser beam at an appropriate wavelength and thereby perform selective multiple-photon excitation of a molecule containing an isotope of interest so as to decompose it. Consequently, the isotope of interest is enriched in either the decomposition product or the yet to be decomposed feed. This is the principle of isotope separation and enrichment on the basis of infrared multiple-photon dissociation.
[Current Status of Laser Isotope Separation and Enrichment of Silicon]
The infrared multiple-photon dissociation of Si2F6 by a one-color pulsed laser beam from a TEA (Transversely Excited Atmospheric pressure) CO2 laser has been studied in great detail (M. Kamioka, S. Arai, Y. Ishikawa, S. Isomura and N. Takamiya, Chem. Phys. Lett., 119, 357(1985); M. Kamioka, Y. Ishikawa, H. Kaetsu, S. Isomura and S. Arai, J. Phys. Chem., 90, 5727 (1986)). According to the results of those studies, the dissociation induced with pulsed laser beam at around 950 cmxe2x88x921 caused 29Si and 30Si to be highly enriched in the gaseous product SiF4 or the solid product (SiF2)m whereas 28Si was highly enriched in the undecomposed Si2F6.
This technique is capable of efficient separation and enrichment of silicon isotopes with a small apparatus. Through combination of appropriate operations including continuous supply of the feed, irradiation with a high-power TEA CO2 laser and continuous recovery of the product and the undecomposed feed, a pilot experiment of separating silicon isotopes has been already performed. A further development effort is anticipated to realize mass supply of silicon isotopes at low cost.
[Problems with the Prior Art Laser Isotope Separation and Enrichment of Silicon]
The RandD activities made to date have revealed that in order to separate and enrich silicon isotopes by means of a one-color laser beam, the applied laser beam must have a wave number around 950 cmxe2x88x921. If the laser beam has a wavelength closer to the absorption band of Si2F6, namely, a larger wave number, the dissociation yield increases but, on the other hand, the isotope selectivity decreases markedly. If the laser beam has a smaller wave number, both the dissociation yield and the isotope selectivity decrease. However, as shown in FIG. 1, the single-photon absorption at around 950 cmxe2x88x921 is very weak for each of the three isotope molecules. One must therefore conclude that the efficiency at which the molecule first absorbs photons is extremely low and so is the efficiency of total process of multiple-photon absorption.
With a view to meeting the demand for realizing large-scale utilization of silicon isotopes in the future, the present inventors made intensive theoretical and experimental work for increasing the efficiency of laser isotope separation and enrichment and successfully developed the following method.
In its basic aspect, the invention solves the above-mentioned problems of the prior art by irradiating Si2F6 synchronously with pulsed beams from two or more TEA CO2 lasers at different wavelengths.
Specifically, the invention provides an efficient method for separation and enrichment of silicon isotopes such as 28Si, 29Si and 30Si on the basis of laser-assisted infrared multiple-photon dissociation of silicon halides, which comprises the steps of emitting laser beams from two or more laser sources at different wavelengths, adjusting the energy of the emitted laser beams by either causing them to pass through CaF2 crystal plates or controlling the discharge voltage across the laser electrodes, and irradiating silicon halides synchronously with the adjusted beams.
The invention also provides an efficient apparatus for separation and enrichment of silicon isotopes such as 28Si, 29Si and 30Si on the basis of laser-assisted infrared multiple-photon dissociation of silicon halides, which comprises two or more laser sources for irradiating silicon halides with laser beams at different wavelengths, CaF2 crystal plates through which the laser beams emitted from laser sources are passed in order to adjust the energy of laser beams, a reactor having NaCl crystal or KCl crystal windows through which the laser beams are launched into the reactor to irradiate silicon halides contained in the reactor, and a delay generator for adjusting the timing of the pulsed beams from two or more laser sources such that they pass through the reactor simultaneously.